Electrically driven wheelchairs of the mid-wheel drive type have recently increased in popularity. Mid-wheel drive wheelchairs normally comprise three pairs of wheels including a pair of front wheels, a pair of rear wheels and a pair of drive wheels, with the drive wheels arranged between the front and rear wheels. A drive unit, normally comprising an electrical motor and a gearbox, is operatively connected to each mid wheel. By controlling the respective drive units electrically, it is possible to impel the drive wheels independently for driving the wheelchair forward, backward and in any desired turning direction.
In comparison to other types of electrical wheelchairs, mid-wheel drive wheelchairs generally provide good stability and a comfortable ride even on uneven surfaces, such as those encountered during out-door driving.
It is of great importance that electrical wheelchairs are stable and not prone to tipping or turning over, and that they provide a comfortable ride on a variety of surfaces. In order to provide good mobility to the person riding the wheelchair, it is of great importance that the wheelchair is able to climb and descend obstacles, such as curbs, doorsteps and the like. It is further desirable that the wheelchairs exhibit a rugged construction with comparatively few components and especially few moving components.
The prior art comprises numerous examples of mid-wheel drive wheelchairs having different types of link arm and suspension arrangements. U.S. Pat. No. 4,513,832 gives an example of a wheeled chassis for a wheelchair. The chassis comprises a first frame structure carrying a first pair of support wheels and a pair of drive wheels and a second frame structure carrying a second pair of support wheels. The first and second frame structures a pivotally connected to each other.
U.S. Pat. No. 7,516,984 B2 discloses a jointed mechanism of an electric wheelchair of the mid-wheel drive type. The jointed mechanism comprises a main chassis assembly, two front caster assemblies, two rear caster assemblies and two drive wheel assemblies which are fixed to a respective of the rear caster assemblies. The front caster assemblies are hinged to the main chassis assemblies. The rear caster assemblies are hinged to the front caster assemblies and fixed to the main chassis by means of a respective pivot arm plate assembly.